When a ferromagnetic compact is magnetized, its dimensions change; this phenomenon is called magnetostriction, and materials that cause this phenomenon are called magnetostrictive materials. The saturation magnetostrictive constant, which is the saturation change amount caused by magnetostriction, generally has the value of 10−5-10−6, and magnetostrictive materials with large saturation magnetostrictive constants are widely used in oscillators, filters and sensors.
At present, magnetostrictive materials with even larger magnetostriction values are demanded, and among the materials proposed are compounds made of rare earth (R) and iron (Fe). R and Fe form RFe2 Laves-type intermetallic compounds, and although the magnetostriction value of RFe2 Laves-type intermetallic compounds is large when the external magnetic field is large, it is insufficient when the external magnetic field is small. Consequently, a magnetostrictive material having an even larger magnetostriction value is in demand among RFe2 Laves-type intermetallic compounds.
As one technique to obtain a larger magnetostriction value for a magnetostrictive material, the density of sintered compact may be increased when the sintered compact is manufactured using the powder metallurgical method. The powder metallurgical method involves heating metal or alloy powder to high temperature and sintering the same, and it is a method to manufacture sintered compacts of magnetostrictive material having predetermined shapes. It is suitable for mass production and offers an advantage of being able to produce a variety of shapes with high yield.
However, the magnetostrictive material manufactured through the powder metallurgical method has gaps among powder particles in the magnetostrictive material and the gaps remain and become pores after sintering, and this phenomenon impede the manufacture of a high density sintered compact. After using the material containing pores as a magnetostrictor for a long time, these pores lead to dry corrosion of the rare earth metal, which oxidizes, especially at high temperature in atmospheric air, and the magnetostrictive properties diminish with these changes.
One way proposed to reduce the number of pores in order to manufacture sintered compacts made of high density magnetostrictors is, for example, (1) to use an argon (Ar) gas atmosphere to sinter when manufacturing a supermagnetostrictor represented by RT2 using the powder metallurgical method (Croat, J. J. “Liquid Sintering of Rare Earth-Iron (Dy0.7Tb0.3Fe2) Magnetostrictive Materials.” J. Appl. Phys. 49.3 (1978)). Also, (2) there has been proposed a method for manufacturing a magnetostrictive material in which several types of raw material powders obtained through machine grinding are sintered in an Ar gas atmosphere, in order to align the crystal orientation of the powders in a compaction process in magnetic field (see Japanese laid-open unexamined Patent Application H 7-286249). Similarly, (3) another manufacturing method proposed is a method for manufacturing a magnetostrictive material in which raw material powder that is composed of plural kinds of raw materials including hydrides of some of the raw materials is sintered in an Ar gas atmosphere.
However, the manufacturing method proposed by Croat yields only about 86% density, which is low, in the sintered compact. Further, even with the method for manufacturing a magnetostrictive material according to the method described in Japanese laid-open unexamined Patent Application H 7-286249, the density of the sintered compact formed from the magnetostrictive material is also low at only about 86%. Moreover, the density of the sintered compact obtained through the manufacturing method described above that uses several types of raw material powders also yields a low density of about 88-93%.
In terms of magnet materials, the following has been proposed: (4) permanent magnets with high coercive force that are manufactured by plastically forming a R—Fe—B system magnet material with a hot press (see Japanese laid-open unexamined Patent Application S 62-202506); (5) anisotropic magnetic powder in which a superplastic metal powder and a pyrolytic binder are added to an anisotropic magnetic powder to form a mixture, the mixture is oriented by a magnetic field, the binder is eliminated through pyrolysis, and the rest is subject to main sintering and hot isostatic pressing (hereinafter called “HIP”) (see Japanese laid-open unexamined Patent Application H 6-192709); and (6) R—Fe—B system permanent magnets, in which permanent magnets are formed by compacting raw material powders through compaction pressure and directly circulating electric current; and raw material powder for permanent magnets that can be compacted to have a high density, which are manufactured by applying relatively a low pressure by means of hot pressing or HIP to the raw material powder (see Japanese laid-open unexamined Patent Application H 10-189319).
However, because the manufacturing methods described above involve sintering in Ar gas, which is an inert gas, the Ar gas fills the closed pores inside the sintered compacts, and when the sintered compacts are HIP-treated and compressed, the internal pressure caused by the Ar gas inside the closed pores leads to a strain; when these methods are applied to magnetostrictive materials, the strain lowers magnetic properties such as magnetostriction value.
Another technique to increase the magnetostriction value of magnetostrictive material involves orienting an RFe2 Laves-type intermetallic compound manufactured under the powder metallurgical method in the direction of a [111] axis, which is an easy axis of magnetization and which provides a large magnetostrictive constant, to thereby obtain a magnetostrictive material whose magnetostriction value is large even when the external magnetic field is small and that has good magnetic field responsiveness.
For example, (7) among the conventional magnetostrictive materials in which crystals are oriented are magnetostrictive materials manufactured through the single crystal method. In addition, there has been proposed (8) a magnetostrictive material oriented along a [111] axis through the powder metallurgical method in which a Tb0.3Dy0.7Fe2.0 powder is compacted in a magnetic field and subsequently sintered (see U.S. Pat. No. 4,152,178). Furthermore, there has been proposed (9) an alloy of Dy, Tb and Fe in which particles of Fe2Tb and Fe2Dy are compacted in a magnetic field into a compression compact and sintered (see Japanese laid-open unexamined Patent Application H 1-180943). Moreover, there is also proposed (10) a magnetostrictive material in which a rare earth-iron with Mn added thereto is used as a basis and the magnetostrictive material is grown in a [110] axis orientation, which is an easy axis orientation for crystals to grow (see Japanese laid-open unexamined Patent Application H 5-148594).
Also, another proposed method relates to (11) a method to manufacture a magnetostrictive sintered compact, in which RFe2 powder and powder of R and Fe eutectic composition that is adjusted using the gas atomizing method or rotating electrode method are mixed, finely ground, compacted in a magnetic field and sintered (see Japanese laid-open unexamined Patent Application H 6-256912). Further, a conventional technology that involves finely grinding with a vibrating mill and sintering is known as a way to achieve high density in a sintered compact.
However, the single crystal method such as the method (7) described above, whether using the zone melting method or the Bridgman method, requires casting the raw material after melting it to form a cast ingot, making a single crystal using the cast ingot, annealing and machining; consequently, its productivity is low and because its shape is limited to a cylindrical shape it requires cutting and other machining to make it into articles. An additional problem with the single crystal method, particularly when using the Bridgman method, is that the single crystal fails to be oriented in the direction of the [111] axis. And the aforementioned method (8) described in U.S. Pat. No. 4,152,178 has a problem of requiring a large magnetic field for orientation to take place due to the fact that the crystal magnetic anisotropy of Tb0.3Dy0.7Fe2.0 is small. Further, the alloy proposed in the aforementioned method (9) described in Japanese laid-open unexamined Patent Application H 1-180943 entails a problem of the constituent metal compounds failing to orient themselves in the direction of the [111] axis, since the easy axis of magnetization for Fe2Tb is the [111] axis, while for Fe2Dy it is the [100] axis. Also, in the aforementioned method (10) described in Japanese laid-open unexamined Patent Application H 5-148594, due to the fact that the crystal grows in the direction of the [110] axis, the magnetostrictive material requires cutting and other machining in order to obtain a magnetostrictive material oriented in the [111] axis, which is an easy axis of magnetization and provides the largest magnetostrictive constant. Further, with the powder obtained through the gas atomizing method in the aforementioned method (11) described in Japanese laid-open unexamined Patent Application H 6-256912 or the powder obtained through the use of a vibrating mill described above, there is a problem in that the sintering density is not necessarily sufficient for obtaining high magnetostrictive properties even though the sintering density is increased.
Furthermore, alloys comprising RFe2 Laves-type intermetallic compounds as described above sometimes precipitate, depending on the alloy composition and/or manufacturing conditions, heterogeneous phases such as phases represented by RFe3, for example, and/or phases formed by impurities in raw material, such as oxides or carbides, in addition to the main phase RFe2. These heterogeneous phases affect the magnetostrictive properties of RFe2 Laves-type intermetallic compounds. Consequently, the precipitation of heterogeneous phases must be controlled in order to obtain superior magnetostrictive properties and to prevent fluctuations in the properties among products, i.e., magnetostrictive materials.
A “super magnetostrictive alloy” described in Japanese laid-open unexamined Patent Application H 5-148594 is an alloy of Fe and R that has been partially replaced with Mn and other metals, and is an alloy containing 5 vol. % or less of the RFe3 phase, which is a heterogeneous phase. By controlling the alloy composition in the super magnetostrictive alloy, the precipitation of the RFe3 phase is restricted and the magnetostrictive properties of the alloy are improved.
However, the aforementioned Japanese laid-open unexamined Patent Application H 5-148594 provides no consideration as to systems in which the amount of rare earth metal represented by R is increased in the alloy composition. Accordingly, it is necessary to study such compositions and find the optimum range.